An ordinary multiple optical axis photoelectric sensor is provided with a light projecting section in which a plurality of light projecting elements are arranged in a row, and a light receiving section in which the same number of light receiving elements as the light projecting elements are arranged in a row. The light projecting elements and the light receiving elements are disposed to face each other in a one-to-one relationship in such a way as to set up a detection area by a plurality of optical axes.
The light projecting section turns on the light projecting elements in sequence. The light receiving section abstracts amounts of light received at respective light receiving elements from the light receiving elements corresponding to respective light projecting elements, at timings that are synchronized with the light emission operation of the light projecting elements. With this configuration, the light blocking states of the optical axes of the multiple optical axis photoelectric sensor are detected respectively. The light receiving section determines whether or not an object is present in the detection area by utilizing the detection results for the respective optical axes, and outputs a signal indicating the determination result. For the purpose of synchronizing the light projecting section with the light receiving section, the light projecting section and the light receiving section are connected to each other by way of a communication line. Alternatively, the light projecting section and the light receiving section may be synchronized with each other by means of optical communication between the light projecting section and the light receiving section.
The multiple optical axis photoelectric sensor is disposed as a device for assuring worker's safety in a production site, for example. Production equipment will stop operating, when a light blocking state is detected in any of the optical axes in the detection area of the multiple optical axis photoelectric sensor. Depending on the production apparatus, there is a case that the detection area of the multiple optical axis photoelectric sensor must be provided in a transport path for passing therethrough workpieces to be processed or already processed workpieces. However, the productivity will be reduced when the production apparatus stops operating because of blocking the optical axis by a workpiece.
Conventionally, as a function for achieving a good balance between safety and productivity in the production site, a muting function has been utilized often for temporarily deactivating the safety function of the multiple optical axis photoelectric sensor. However, in recent years, with the advancement in usage, productivity and safety are required for complicated workpiece forms, conventional evenly shaped workpiece forms and combinations thereof, for example.
In general, the muting function deactivates the entire detection area only when two muting signals independent from sensors and switches are output in an appropriate sequence. Therefore, the conventional muting function generally deactivates the entire detection area including other areas than the area to be deactivated, leading to a non-preferable state.
For example, JP 2003-218679A discloses a multiple optical axis photoelectric sensor aimed to achieve a good balance between a light blocking object detection function and a muting function. This multiple optical axis photoelectric sensor is provided with a muting area setting means for setting, by teaching, an area where the muting function is activated. The muting area setting means activates the muting function only to some areas of a light curtain.
In the multiple optical axis photoelectric sensor disclosed in JP 2003-218679A, when the height (size) of workpieces passing through the detection area is constant, it is possible to set up the area where the muting function is activated in accordance with the height of the workpiece. However, in applications where the height of workpieces varies, it is necessary to set up the area for the activation of the muting function in accordance with the maximum height of the workpieces.
Therefore, if the height of the workpieces passing through the detection area varies, there is still a problem of the deactivation of other areas than the areas intended to be deactivated. For example, such a problem may arise in case of conveying pallets carrying products together with pallets not carrying products. JP 2003-218679A discloses a feature of providing a plurality of muting areas in advance in the multiple optical axis photoelectric sensor for allowing users to set the muting area. However, it is difficult to adapt the muting area to the workpieces in accordance with the setting in case that workpieces having different heights are conveyed together.
JP-2010-385588A discloses a multiple optical axis photoelectric sensor that is switchable between two muting areas and a controller for the same. The controller stores the two muting areas in advance. The controller switches the muting areas in response to a detection signal output from a sensor. In this instance, a sensor is required for switching the muting areas. In addition, it is necessary to store a plurality of muting areas in advance.
JP-2003-218679A and JP-2010-385588A are examples of related prior art.